You can utilize the rule condition "If a task falls during a holiday or time off..."

This rule condition allows you to take an action when a task gets scheduled during a holiday or a user's scheduled time off.

Whenever a task is assigned to someone in the target project, or an assigned user's task in that project has its start/due dates changed, this rule will be evaluated, and if the assigned start/due dates fall within one or more time-off entries in the "holiday/time off calendar", the rule's action will be taken.

You must create a project that holds the holiday/time-off information and then select that "holiday/time off calendar" project when creating this rule condition in a target project. 

Here are the steps to create the project which holds the holiday/time-off information - we'll call it the "holiday/time off calendar" project:
  1. Create the project and name it whatever you choose. It can reside anywhere in your Asana instance that you have access to.
  2. In the project, create or add one and only one people-type custom field. You can name this field whatever you choose (we've named it "Employees" in the example below). This field is used to identify which user or users a particular time-off entry applies to.
  3. Add tasks in the project for each time-off entry, as follows:
    • You can name the tasks whatever you choose.
    • Set the task's due date (if it's a single-day event, like a holiday), or its start and due date (if it's a multi-day event, like a user's vacation).
    • In the people-type custom field, select the user or users that this time-off applies to. You can also leave this custom field blank, in which case it will apply to any user in the project. (This is useful for company-wide holidays, for example.)
Here's an example of what this "time off calendar" project might look like:



Other than the above requirements, you can set up this project however you'd like - for example, you can group the tasks into sections, you can add any other custom fields you choose to (as long as they are not people-type fields), etc.

Don't forget, you must select the above "time off calendar" project when creating this rule condition in a target project:


Two useful actions you might want to use in combination with this trigger condition are Add a comment... or Send an email notification... to alert the user that a task has been assigned during their time off. Because you can use Flowsana's Variable Substitution in the comment or email notification, you can include details about the offending task, its dates, etc. Note that if you use {task.Assignee} in the Add a comment... action, Flowsana will convert that to an @mention of that user.

NOTE: If you prefer, you can use the Assignee field instead of a people-type custom field to identify the user that a given time-off date or date range applies to. To use the Assignee field, simply omit any people-type custom fields from the "time off calendar" project; if Flowsana does not find any people-type custom fields in that project, it will automatically use the Assignee field instead.

The disadvantage to using the Assignee field is that it only holds one user, so you'll need to replicate a time-off event if you need to apply it to multiple users. The advantage is that it will show in users' My Tasks so their upcoming time off is visible.

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We know that some people would also like to be able to tell Flowsana to skip over holidays/time off on Dynamic Duration and Auto-Adjust workflows, and we plan to provide this capability as a product enhancement in the future.